1. Field of the Invention
This invention relates to a multi-axis numerically-controlled machine, which can be used to produce a surface of any shape whatsoever required and of finite extent on a workpiece, of the type comprising a workpiece holder to which a workpiece can be attached and which is rotatably mounted around a first geometrical axis, a first driving means connected to the workpiece holder for rotating it around the first geometrical axis, a rotating tool for machining a workpiece attached to the workpiece holder, said tool having an active surface which is a surface of revolution around an axis of rotation of the tool, a second driving means connected to the tool for rotating it around its axis of rotation, a first carriage which supports one of the two elements, i.e. the workpiece holder and the tool, and which is linearly movable along a second geometrical axis parallel to the first geometrical axis, a third driving means connected to the first carriage for displacing it along said second geometrical axis, a second carriage, which supports one of the two above-mentioned elements and is linearly movable along a third geometrical axis perpendicular to the first geometrical axis and secant with it, a fourth driving means connected to the second carriage for displacing it along said third geometrical axis, said surface of revolution of the tool having a center which is at all times contained in the plane defined by the first and third geometrical axes, control means connected to said first, third and fourth driving means for displacing the tool and the workpiece holder in relation to one another in response to data corresponding to a required surface to be produced on said workpiece. The invention also relates to a machining method using such a machine.
2. Description of the Prior Art
Machines of this type are already known (see European patent application No. 0,281,754, for instance). The machines of this type can notably be used for producing aspheric, concave or convex surfaces, e.g. surfaces referred to as "progressive surfaces" or variable power surfaces, either directly on an ophthalmic lens blank, or on a block of material suitable for subsequent use as sagging shape for the manufacture of an ophthalmic lens by thermal sagging, or even on a block of material suitable for subsequent use as a mold for the manufacture of an ophthalmic lens by molding of an organic material.
The known machine described in the above-mentioned European patent application is a three-axis machine, i.e. a machine in which, in addition to the rotating movement proper of the tool around its axis of rotation, there are three axes of relative movement between the tool and the workpiece holder or blank holder, namely a rotating movement of the workpiece holder around a first geometrical axis, a translational motion of the workpiece holder, borne by a first carriage, along a second geometrical axis parallel to the first geometrical axis, and a translational motion of the tool, borne by a second carriage, along a third geometrical axis perpendicular to and secant with the first geometrical axis. In this known machine, the three above-mentioned movements are used to achieve both the required spiral path on the surface of the workpiece and the depth of cut or thickness removed, i.e. the quantity of material that the tool must remove in sequence at points successively spaced along said spiral path. As a result, the carriage bearing the workpiece holder must perform an oscillating rectilinear movement, of which the amplitude of oscillation can reach relatively high values, notably in the case where the optical surface to be machined has radii of curvature with very different values in the equator plane and the main meridian plane of the surface to be machined, i.e. each time that by successive rotations of the workpiece holder through 90.degree. around the first geometrical axis the point of contact between the tool and the surface to be machined moves from the equator plane to the main meridian plane, then back to the equator plane, and so forth. Such an operating mode is unfavorable from the point of view of machining precision, machining time and with regard to the freedom of choice of the machining path or other machining parameters. As regards machining precision and time, it should be noted that these two values are closely related. Indeed, the slower the displacement or oscillation speed of the tool or workpiece holder, the longer the machining time, but machining precision increases since the tracking errors generated by position-controlling devices (numerical control) associated with the various axes of movement are smaller.